Wheelchairs are used by people for whom walking is difficult or impossible due to illness (physiological or physical), injury, or disability. Conventional wheelchairs are propelled either by a motor or manually (e.g., by the seated occupant turning the rear wheels by hand).
A basic manually-operated wheelchair incorporates a seat, foot rests, and four wheels: two caster wheels at the front and two larger wheels at the back. The two rear wheels are used to propel the chair. Turning both of the rear wheels simultaneously in the same direction (e.g., by the seated occupant using both hands) propels the chair in a straight line, forward or backward. Turning only one of the wheels (e.g., by the seated occupant using only one hand) results in a pivot or circular propulsion of the wheelchair. It is typically not possible to propel a conventional manually-operated wheelchair in a straight line using only one hand.
Unfortunately, the seated occupant may not have easy use of both hands. For instance, certain illnesses or injuries (e.g., cancer, multiple sclerosis, amputation) may limit an individual's use of his or her hands and arms. This makes operation of a manually-operated wheelchair difficult, since both hands are typically needed in order to enjoy the full range of motion of the chair as discussed above.